Inactivation of p53 is a common event in the development of human neoplasia (Hollstein et al. (1991) Science 253, 49-53). A variety of mechanisms can lead to such functional inactivation, including p53 point mutations of deletions of p53 (Baker et al. (1989) Science 244, 217-221; Wolf, D., and Rotter, V. (1985) Proc. Natl. Acad. Sci. USA 82, 790-794), and interaction with oncogenic viral or cellular proteins (Mietz et al. (1992) EMBO J. 11, 5013-5020; Momand et al. (1992) Cell 69, 1237-1245). Wild-type p53 has been shown to be a suppressor of tumor cell growth (for reviews see Mercer, W. E. (1992) Crit. Rev. Eucar. Gene Exp. 2, 251-263; Oren, M. (1992) FASEB J. 6, 3169-3176; Lane, D. P. (1992) Nature 358, 15-16; Perry, M. E., and Levine, A. J. (1993) Curr. Opin. in Genet. and Devel. 3, 50-54). Inactivation of p53 by any of the above mechanisms thereby leads to a selective growth advantage, generally observed as tumor progression.
The mechanism underlying p53 growth suppression is still undefined. Several biochemical features of p53 have been elucidated, and at least two of these are currently of much interest. First, p53 has been shown to transcriptionally suppress a variety of promoters containing TATA-elements (Ginsberg et al. (1991) Proc. Natl. Acad. Sci. USA 88, 9979-9983; Santhanam et al. (1991) Proc. Natl. Acad. Sci. USA 88, 7605-7609; Kley et al. (1992) Nucl. Acids Res. 20, 4083-4087; Mack et al. (1993) Nature 363, 281-283). This suppression is apparently sequence independent, and may involve p53 binding to tie TATA-binding protein (TBP) or to other transcription factors (Seto et al. (1992) Proc. Natl. Acad. Sci. USA 89, 12028-12032; Truant et al. (1993) J. Biol. Chem. 268, 2284-2287; Ragimov et al. (1993) Oncogene 8, 1183-1193; Martin et al. (1993) J. Biol. Chem. 268, 13062-13067; Liu et al. (1993) Mol. and Cell. Biol. 13, 3291-3300). Second, p53 can bind to DNA in a sequence-specific manner (Kern et al. (1991) Science 252, 1707-1711). A 20 bp consensus binding site, consisting of two copies of the 10 bp sequence 5'-RRRCWWGYYY-3', separated by up to 13 bp, has been identified (El-Deiry et al. (1992) Nature Genet. 1, 45-49; Funk et al. (1992) Mol. Cell. Biol. 12, 2866-2871). Both copies of the 10 bp sequence are required for efficient binding by p53. p53 contains a strong transcriptional activation sequence near its amino terminus (Fields, S., and Jang, S. K. (1990) Science 249, 1046-1049; Raycroft et al. (1990) Science 249, 1049-1051), and can stimulate the expression of genes downstream of its binding site. Such stimulation has been demonstrated in both mammalian (Kern et al. (1992) Science 256, 827-830; Funk et al. (1992) Mol. Cell. Biol. 12, 2866-2871; Zambetti et al. (1992) Gen. and Devel. 6, 1143-1152) and yeast cells (Scharer, E., and Iggo, R. (1992) Nucl. Acids Res. 20, 1539-1545; Kern et al. (1992) Science 256, 827-830) as well as in an in vitro system (Farmer et al. (1992) Nature 358, 83-86).
The sequence-specific transcriptional activation by p53 has led to the hypothesis that p53-induced genes may mediate its biological role as a tumor suppressor (Pietenpol et al. (1993) Cell (submitted)). To date, several genes containing p53-binding sites have been identified. These include muscle creatine kinase (M C K, Weintraub et al. (1991) Proc. Natl. Acad. Sci. USA 88, 4570-4574; Zambetti et al. (1992) Gen. and Devel. 6, 1143-1152), GADD45 (Kastan et al. (1992) Cell 71, 587-597), MDM2 (Barak et al. (1993) EMBO 12, 461-468; Wu et al. (1993) Genes and Devel. 7, 1126-1132), and a GLN retroviral element (Zauberman et al. (1993) EMBO J. 12, 2799-2808). Each of these genes contains a 20 bp sequence with high homology to the p53 consensus binding site (Prives, C., and Manfredi, J. J. (1993) Gen. and Devel. 7, 529-534). The p53-binding sites in GADD45 and MDM2 are located within introns, the MCK site is 3 kb upstream of the transcription start site, and the GLN element is located within an LTR. The relationship of any of these genes to suppression of cell growth by p53 remains unclear. It has been suggested that MDM2 may be a feedback regulator of p53 action, by being transcriptionally induced (Barak et al. (1993) EMBO 12, 461-468; Wu et al. (1993) Genes and Devel. 7, 1126-1132), then inhibiting p53 function (Momand et al. (1992) Cell 69, 1237-1245; Oliner et al. (1993) Nature 362, 857-860; Wu et al. (1993) Genes and Devel. 7, 1126-1132). In this regard, MDM2 functions as an oncogene rather than as a tumor suppressor gene (Fakharzadeh et al. (1991) EMBO J. 10, 1565-1569; Finlay, C. A. (1993) Mol. and Cell. Biol. 13, 301-306).
There is a need in the art for elucidation of the pathway by which p53 exerts its tumor suppressive effects. There is also a need in the art for new diagnostic and therapeutic tools for evaluating and ameliorating human cancers.